The present invention generally relates to semiconductor devices and processes for producing semiconductor devices. In particular, the present invention relates to a process for producing a semiconductor device, the process including forming a pattern by printing. The present invention also relates to a semiconductor device produced by the process.
In recent years, semiconductor devices including active layers containing organic semiconductor materials have been receiving attention. In such a semiconductor device including the organic semiconductor material, it is possible to form the active layer composed of the organic semiconductor material by application at a low temperature. Thus, the semiconductor device including the organic semiconductor material has an advantage in view of cost reduction. Furthermore, the semiconductor device can be formed on a low-heat-resistant flexible substrate, such as a plastic substrate. Moreover, a gate insulating film, a source electrode, a drain electrode, and a gate electrode in addition to the active layer can be formed by patterning using printing with application materials, thus leading to further cost reduction.
An inkjet printing has been studied as a method for forming a pattern with such an application material. Various materials can be applied by the inkjet printing as long as the materials each have a viscosity of several centipoises. However, in the inkjet printing, in view of the difficulty in controlling the amount of ink discharged and the precision of the position into which the ink is discharged, printing precision is about 20 μm at the present time. Thus, to increase the precision, a method for forming banks composed of polyimide or the like at the periphery of a position into which the ink is discharged has been proposed.
On the other hand, in addition to such an inkjet printing, a method for forming a fine pattern by printing, such as screen printing, which uses a template (screen), has been studied. Among printing with such a template, a nanoimprinting in which a fine structure is formed by pressing a stamp having a relief pattern against an uncured film formed by application is described by Michael D. Austin and Stephen Y. Chou [Appl. Phys. Lett., Vol. 81, 4431 (2002)] (Non-Patent Document 1). A method of using an elastomeric stamp having a relief pattern is proposed in PCT Japanese Translation Patent Publication No. 2003-509228 (Patent Document 1). Furthermore, microcontact printing using a stamp made by transferring a fine pattern formed by lithography into an elastomeric plastic is proposed in A. Kumar, G. M. Whiteside et al. [Langmuir, Vol. 10, 1498 (1994)].
However, in applying the above-described printing methods to production processes of semiconductor devices, there are problems described as follows.
In forming a fine pattern by inkjet printing, it may be essential to form banks at the periphery of a region into which the ink is discharged, as described above. Thus, it may be necessary to perform many additional steps, such as an applying step of applying a material constituting the banks, for example, polyimide, and a patterning step of patterning the applied film by photolithography, thereby disadvantageously complicating the production process.
In screen printing, it is difficult to form a film having a thickness of 1 μm or less. Thus, for example, when an active layer pattern composed of an organic semiconductor material is formed by screen printing, a step height of 1 μm or more occurs at the surface. Therefore, when a multilayer interconnection is formed on the active layer pattern, a portion not covered with an interlayer insulating film easily occurs at a side wall of the step, thereby possibly causing a short circuit between an upper lead and an lower lead.
In contrast, in nanoimprinting and microcontact printing, it is possible to form a fine pattern having a step height of 1 μm or less. However, in these printing methods, there are limits to the compatibility (adhesion) between an application material and a stamp and between the material and a substrate on which a fine pattern is formed by printing. Thus, any material cannot be always used for pattern formation by these printing methods.